In the petroleum refining industry, a heavy oil feed stock of high boiling hydrocarbons is contacted at elevated temperatures with a fluidized solid catalyst in a reaction zone to effect conversion or "cracking" of at least a portion of the feed stock to lower boiling hydrocarbon products, such as gasoline. In modern cracking equipment, the reaction zone usually comprises a vertical pipe or riser into which is fed a mixture of hot catalyst and heavy oil feed stock. The riser ends in a reactor vessel where the hydrocarbon materials are rapidly separated from the catalyst to abruptly terminate the conversion reaction. During the conversion reaction, carbon in the form of coke is deposited on the solid catalyst particles and inhibits their catalytic activity. To maintain the effectiveness of the catalyst, coke contaminated catalyst is first stripped of residual hydrocarbons and then transferred to a fluidized bed regenerator where the catalyst is contacted with an oxygen containing gas at sufficiently high temperatures to burn off the coke and thereby regenerate the cracking activity of the catalyst. The high temperature required to burn the coke and the heat required for the conversion reaction are both supplied, at least in part, by the carbon combustion reaction which yields carbon oxides as combustion products. Both the oxygen containing gas and the combustion products will be referred to here as combustion gases. Since combustion gases are undesirable in the reaction products from the riser, regenerated catalyst is sometimes also stripped to remove those gases prior to returning the regenerated catalyst to the riser.
Spent catalyst is continuously withdrawn from the reactor through a carrier line and transferred to a stripper and then to a spent catalyst standpipe. Regenerated catalyst is continuously withdrawn from the regenerator and returned to the riser via a regenerated catalyst standpipe which may also be preceded by a carrier line and/or a regenerated catalyst stripper. Both the regenerator and the strippers usually contain a fluidized bed with a dense lower phase and a dilute upper phase. The dilute phase is comprised primarily of combustion gases or stripping media, together with smaller amounts of entrained catalyst, and the dense phase is comprised of solid catalyst particles with sufficient fluidizing gas or vapor to maintain the solid catalyst particles in a fluid-like state. Similar to the molecules of a liquid, the catalyst particles in the dense phase have a net downward component sufficient to produce gravity flow of the fluidized catalyst. Although the fluidizing and stripping medium in the stripper may be either an inert gas or an inert vapor, such as steam, it will be referred to generally as a stripping gas.
As used hereinafter, the term "fluidized bed" will refer only to the dense catalyst phase. Although many catalytic cracking units also employ a fluidized bed in the reactor, the use of extended risers within the reactor vessel has recently led to elimination of reactor fluidization in some of these units. Instead, catalyst leaving the riser falls into a loose bed at the bottom of the reactor. This non-fluidized bed is sufficiently dry and loose for the catalyst particles to flow by gravity into the next conduit or vessel. Thus, the reactor catalyst flows as a loose stream of particles down the carrier pipe into the spent catalyst stripper where it becomes fluidized by the stripping gas to form a fluidized bed only within the stripper itself. However, the diameter of the carrier pipe between the reactor and the stripper is often the same as that of the stripper and the top of the fluidized stripper bed may actually extend up into the carrier pipe, although the top of the bed is preferably maintained near the top of the main stripper section. By comparison, the catalyst medium seen by the regenerated catalyst standpipe, and the regenerated catalyst stripper where used, is a continuation of the fluidized bed maintained in the regenerator so that these latter components are generally at a greater bed depth where they are subjected to a higher fluid head.
The vessels and conduits used in a catalytic cracking unit or system are usually cylindrical in shape and of varying diameters, with the reactor vessel and the regenerator vessel having the largest diameters and the spent catalyst and regenerated catalyst standpipes having the smallest diameters. The strippers and related transfer or carrier conduits are usually of a diameter intermediate between the regenerator and reactor vessels and the standpipes. Whereas the heights of these components vary relatively little between units of different capacities, their diameters can vary widely depending on the overall catalyst circulation rate for which the unit is designed and the optimum mass flow density to be achieved in each component. The transfer of catalyst between the reactor and the regenerator therefore involves a relatively large flow of fluidized solids across transitions between vessels and conduits having significantly different diameters. For purposes of clarity, a smaller component receiving flow from a larger component will be referred to as a conduit and the larger component as a vessel. However, catalyst is conveyed by both vessels and conduits and these two terms are considered interchangeable in describing the invention. For example, the stripper performs both a conveying function and a stripping function and can be referred to as either a vessel or a conduit depending on the flow transition being discussed. These definitions are therefore not intended to be restrictive in any way.
The flow of fluidized solids from a larger vessel to a smaller conduit is hindered by the change in flow cross-section experienced by this moving mass of material. Because the same mass of catalyst per unit of time is transferred through both the vessel and the conduit, the mass velocity of the flowing stream increases markedly as the catalyst enters the conduit. This change in mass velocity, together with the flow resistance generated by the sharp edges of the vessel to conduit transition, produces a pressure drop across the outlet at the upper end of the conduit. In this environment, fluidizing gas from the vessel is entrained with the down flowing catalyst and then subsequently released in the conduit. The entrained gas is in excess of that required for fluidization and produces bubbles of catalyst free gas which can flow countercurrent to the catalyst back into the vessel or concurrent with the catalyst through the slide valve. The countercurrent flow of released gas within the conduit can produce flow restriction phenomena known as slugging and bridging. Slugging is a surging of fluidized material in the conduit caused by large bubbles of upflowing gas. Bridging is a complete loss of catalyst fluidity either in the conduit or across the opening of the rim defining the outlet between the vessel and the conduit. Slugging seriously hampers flow. Bridging can be of sufficient severity to stop flow through the conduit entirely. Flow stoppage may require taking the reactor off stream until the plug of unfluidized catalyst can be eliminated and catalyst flow through the conduit reestablished. Bridging of sufficient severity to stop catalyst flow seems to occur more frequently during unit start up before full operating conditions have been attained in the various components. This may be because flow patterns are relatively weak at these times and easily disrupted by flow resistance at the vessel-to-conduit transitions. Concurrent flow of released gas is also undesirable because it produces unstable pressure differentials and high levels of erosion at the slide valve.
The prior art contains patents disclosing withdrawal conduits for catalyst regenerator vessels where the conduit is extended up into the fluidized bed above the solids inlet and cut at an angle to the horizontal. Thus, U.S. Pat. No. 3,964,876 to James and U.S. Pat. No. 2,900,329 to Osborne et al. show overflow-type standpipes for regenerated catalyst where the open end has been cut on a diagonal to form an overflow weir. The solids outlet of these standpipes is therefore at the top of the fluidized bed where catalyst overflows the lower side of the opening but not the upper side to regulate the height of the catalyst bed. Since the gaseous phase of the regenerator has direct access to the standpipe, this arrangement can increase the amount of gas entrained with the catalyst and carried into the standpipe, a result to be avoided according to the present invention. In this regard, the express purpose of the beveled opening to the fluent tube 50' in U.S. Pat. No. 3,677,716 to Weber et al. is so that gaseous effluent may pass directly into the exit tube.
U.S. Pat. No. 4,138,219 to Colvert et al. shows regenerated catalyst withdrawal conduits extended into the fluidized bed up to the level of the solids inlet. In this device, the conduit opening is beveled so that its high side prevents flow directly from the inlet and catalyst must traverse the periphery of the regenerator wall and enter the opening over its low side. U.S. Pat. No. 3,394,076 to Bunn, Jr., et al. shows standpipe apertures facing away from the solids inlet for the same purpose as the beveled openings of Colvert et al. The standpipes of this latter reference also have a top opening in communication with the top of the bed to perform the overflow function previously described.
In the older units such as those above, slugging and bridging phenomena occurred primarily at the stripper to standpipe transition rather than in the regenerator withdrawal conduit which extended high into the fluidized bed so that the solids outlet opening was either at or above the solids inlet opening. The depth of catalyst bed above such prior art outlet openings is relatively shallow. In some of the more recent regenerator designs, however, the extension of the standpipe into the regenerator vessel has been eliminated so that the regenerated catalyst outlet is at the bottom of the vessel. This was done, at least in part, to make bed height relatively independent of the position of the solids outlet. Placement of the catalyst outlet at the bottom of the regenerator vessel resulted in an increase in the bed height above the outlet opening and a corresponding increase in fluid head and bed density in and around this opening. Instead of the shallow, low density bed of older designs, the bottom outlets see a relatively deep, high density bed of fluidized catalyst. The bottom outlet arrangement for fluidized regenerators is similar to the bottom outlet between a catalyst stripper and its associated standpipe. Stripper outlets have long been subjected to a relatively deep and dense bed of fluidized catalyst. This high bed density is believed to contribute to the slugging and bridging phenomena observed at the stripper to standpipe transition. Similar flow resistance can occur in regenerator withdrawal conduits with bottom outlets.